The present invention relates to a lithographic apparatus such as an offset printing machine comprising a novel and unique master sheet feed device.
Such an apparatus generally comprises an etching unit for etching a master sheet and a master drum for windingly supporting the master sheet for printing.
A major problem which has remained heretofore unsolved in the prior art is to devise a convenient manner of feeding the master sheet from the etching unit to the drum. Since a clamp is provided on the master drum to clamp the leading edge portion of the master sheet, the timing of feeding the master sheet to the drum must be exact. An error in timing will result in the sheet not being fed into the clamp but being fed in an erroneous manner to jam between the master drum, transfer drum, etc.
The problem is compounded by the fact that the speed of the master sheet being fed through the etching unit is generally different from the surface speed of the master drum. It has been practiced in the prior art to provide a conveyor belt between the etching unit and the master drum to accomodate the speed difference. However, such a conveyor belt serves to increase the overall size of the apparatus to an unacceptable extent.
It is also known to temporarily synchronize the speeds of the etching unit drive means and the master drum. This expedient requires an electronic speed control means and/or a mechanical speed converter such as a transmission. Both of these devices are expensive and undesirably increase the overall cost and complexity of the apparatus. Even if such a device is provided it does not overcome the problem of feed timing.